Maps showing the 2 ship multibeam bathymetry surveys in the area of the
newly-discovered lava flows.

The data on the left were collected in
2013, the data on the right in 2015.

The black polygons are the new lava
flows. Numbers on the 2015 map specify the maximum thickness of each
lava flow in meters.

The white box shows the area of the Sentry
bathymetry survey.

Doing the Math

This is where mapping we have done with R/V Falkor’s multibeam
sonar came into play.
Sonar systems send sound pulses from the ship to
the seafloor (called “pings”).
Present-day sonar systems can provide up
to 432 individual depth readings (“soundings” or “beams”) across each
ping - this is where the term “multibeam” gets its name.
The amount of
time it takes each sound pulse to return to the ship indicates the depth
beneath the ship: the longer it takes the sound to return, the deeper
the water.
Those depth soundings are pulled together to create a grid of
the seafloor.
Ship-acquired seafloor depth information is scarce and
expensive to acquire, so whenever the opportunity presents itself
scientists will collect sonar data - even if they have no plans to use
it for their individual research.
One aspect of my job is to compile all
the ship sonar data collected in the areas where we work and pull it
together into a database.
Because of this, we have with us the multibeam
sonar data collected on a previous cruise to this area in 2013 (by
Patrick Shore and Doug Wiens on R/V Melville), and we’ve
resurveyed the same area during this expedition.
By performing a little
bit of math, we can calculate the difference in the seafloor depths
between then and now.
The earlier grid (2013) is subtracted from the new
data (2015) and depth differences in those grids show changes in the
seafloor between now and then.

The Results

Surface differencing is a quick way to show us where the new lava
erupted on the seafloor, and we can also calculate the volume of new
lava flows.
The results reveal that the new lava flows extend over 7
kilometers from north of south, and that greater than 60,000,000 cubic
meters of lava was deposited on the seafloor.
The northernmost lava flow
was the largest, with a volume of over 20,000,000 cubic meters, piling
up pillow lavas over 125 meters (400 feet) thick.
Now that’s the kind of
math that I find incredibly exciting!